Commitments and Contingencies - Self Insurance (Details)	Dec. 31, 2017 USD ($) item
Workers Compensation
Self-Insurance
Per incident deductible amount	$ 1,000,000
Amount of loss fully insured above per incident deductible amount.	1,000,000
Employer's Liability
Self-Insurance
Per incident deductible amount	1,000,000
Employer's Liability | Maximum
Self-Insurance
Amount of excess loss insurance covered	100,000,000
General Liability
Self-Insurance
Per incident deductible amount	1,000,000
Amount of loss fully insured above per incident deductible amount.	1,000,000
General Liability | Maximum
Self-Insurance
Amount of excess loss insurance covered	100,000,000
Auto Liability
Self-Insurance
Per incident deductible amount	500,000
Amount of loss fully insured above per incident deductible amount.	1,500,000
Auto Liability | Maximum
Self-Insurance
Amount of excess loss insurance covered	$ 100,000,000
Employee Medical
Self-Insurance
Number of medical plans | item	3
Employee Medical- Plan One
Self-Insurance
Per person, per policy deductible amount	$ 350,000
Employee Medical- Plan Two
Self-Insurance
Per person, per policy deductible amount	$ 350,000